1. Field of the Invention
Embodiments of the present invention generally relate to thermal processing of a substrate and more particularly to method and apparatus for reducing patterning effects on a substrate during radiation-based heating.
2. Description of the Related Art
Some methods of thermal processing of substrates, such as rapid thermal processing (RTP) or laser thermal processing (LTP), rely on radiation-based heating of substrates. For example, RTP is performed on a silicon wafer for the manufacture of integrated circuits by heating a wafer using high-output lamps, such as halogen lamps. The wafer is heated for a short time to a peak temperature, often on the order of 800 to 1000 degrees Celsius, for the formation of a layer of oxide, nitride, or silicide on the wafer, or to anneal an implanted wafer.
Substrate temperatures achieved in radiation-based heating strongly depend on the amount of incident radiation absorbed by the substrate. Even with completely uniform irradiance of a substrate, regions of the substrate surface having different reflectivities will experience different amounts of energy input per unit surface area, and consequently will heat up to different temperatures during the thermal process. Different reflectivities are typically present on a substrate surface because substrates that undergo radiation-based heating processes generally have a heterogeneous surface consisting of multiple material and feature types, rather than a surface covered by a uniform blanket film. This variation in reflectivity and heating rate is known as a patterning effect.
Heterogeneous substrate surfaces that generate such thermal patterning effects are produced by the disparate materials used in the construction of integrated circuits. For example, the surface of an integrated circuit (IC), also referred to as an “IC die” prior to being cut from a silicon wafer, may include a number of different materials during thermal processing, such as silicon oxides, silicon nitrides, metal silicides, and metals. Each material may have different optical properties, e.g., reflectivity, absorptivity, and emissivity, and therefore each material will absorb incident radiation at different rates. The mass of such surface materials is very small compared to mass of the underlying substrate, therefore the heat capacity of every region of the substrate is essentially the same. But because energy input can vary substantially between different regions of an IC die, significant temperature gradients may occur therebetween during thermal processing, for example on the order of 10's or even 100's of degrees Celsius. Conductive heat transfer from “hot spots” to “cold spots” on the substrate generally cannot adequately ameliorate these gradients due to the short duration of radiation-based heating processes.
When a light ray encounters a material, it is either reflected off the surface of the material, absorbed by the material, or transmitted through the material. Therefore, the fraction of radiative energy absorbed by the material is equal to (1—fraction of energy reflected—fraction of energy transmitted). In the case of a silicon-based substrate which is at or above about 500 degrees Celsius and which is illuminated with infra-red (IR) radiation, the energy absorbed by the substrate is simply equal to (1—fraction of energy reflected). This is because silicon-based substrates, such as silicon wafers, are essentially opaque to IR radiation at this temperature, i.e., incident IR radiation is either reflected or absorbed by the substrate and none is transmitted. The fraction of energy absorbed by the substrate is known to be a function of wavelength, polarization, and incidence angle of the illuminating radiation.
FIG. 1A is a graph illustrating the dependence of the energy absorption of a material on the wavelength and incidence angle of illuminating radiation. In this example, the material in question is a film stack deposited on a silicon wafer consisting of a 57 nm film of polysilicon on a 170 nm film of silicon dioxide. A family of absorption curves 110 consists of absorption curves 111-115, each of which represents one absorption curve for this film stack. As used herein, “absorption curve” is defined as the fraction of incident energy absorbed by a material described as a function of the wavelength of the incident energy.
Absorption curves 111-115 represent the different absorption curves for radiant energy that is incident to the absorbing material at 0°, 22.5°, 45°, 67.5°, and 85°, respectively, wherein 0° is defined as normal to the absorbing material. The weighted average fraction of energy absorbed by the material, i.e., the effective absorptivity of the material, may be quantified by integrating each of the absorption curves 111-115 over wavelength weighted by the source spectrum to produce a total absorbed energy for each angle of incidence. The absorbed energy for each curve is then is weighted based on the angular distribution of the incident energy, and the weighted energy totals for each absorption curve are then summed. For example, for a highly uni-directional and normally incident radiation source, such as a laser beam oriented normal to a material surface, absorption curve 111 is weighted to contain 100% of the energy absorbed. This is because absorption curve 111 corresponds to the absorption curve for normal incidence. For more diffuse radiation sources, the absorption curves 112-115, which represent the absorption of radiation at non-normal angles of incidence, are weighted appropriately to accurately reflect the directionality of the radiation source.
FIG. 1B illustrates the spectral power distribution curve 163 for a typical continuum radiator, also referred to as a radiation source, at a standard operating temperature. The abscissa represents the wavelength, in micrometers (μm), of the radiation produced by the continuum radiator. The ordinate represents the exittance of the continuum radiator, i.e., the power per unit area of the radiation source per wavelength of the incident radiation. As can be seen in FIG. 1B, the majority of energy produced by the radiation source occurs in the IR bandpath, i.e., the spectrum of wavelengths between about 0.2 μm and about 4.0 μm. It is noted that the energy so produced is distributed over a continuum of frequencies rather than concentrated at a few discrete frequencies. Further, the spectral power distribution for this continuum radiator is not flat, i.e., more energy produced by the radiator is concentrated in certain wavelengths over other wavelengths, producing a non-uniform spectral power distribution. In the example shown in FIG. 1B, energy output of the continuum radiator is much higher in a peak region 160 of the spectrum than in tail regions 161, 162. Such a non-uniform spectral power distribution is typical for continuum radiators. A common continuum radiator used for radiation-based thermal processing is a tungsten halogen lamp.
Referring back to FIG. 1A, it is noted that, for any given material or film stack, family of absorption curves 110 will be different, i.e., the weighted average fraction of energy absorbed by each material is unique. This difference in absorptive behavior between different materials is a root cause for the patterning effects described above. FIG. 1C schematically illustrates a partial cross-section of a silicon substrate 120 having a source/drain region 121 formed on surface 122 and consisting of crystalline silicon. Silicon substrate 120 also has a film stack region 123 formed on surface 122 and adjacent source/drain region 121, wherein film stack region 123 consists of a polysilicon/silicon film stack as described above in conjunction with FIG. 1A and family of absorption curves 110. Because source/drain region 121 and film stack region 123 are very small features adjacent each other on silicon substrate 120, they receive an essentially identical angular distribution of incident energy 125 during radiation-based thermal processing. Total energy absorbed by source/drain region 121 and film stack region 123 may be substantially different, however. This is because the family of absorption curves for source/drain region 121 may be significantly different than family of absorption curves 110 for film stack region 123. For example, referring back to FIG. 1A, family of absorption curves 110 indicates a low absorption bandpath 116 for wavelengths between about 0.75 μm and about 1.5 μm for film stack region 123. If, in contrast, source/drain region 121 absorbs a higher faction of incident energy over the same wavelengths, source/drain region 121 may absorb much more energy than film stack region 123 during radiation-based heating and therefore experience higher temperatures.
For an integrated circuit or other electronic device undergoing thermal processing, it is desirable for every region of the device to experience the same thermal process, particularly temperature. Thermal gradients across a device caused by patterning effects can result in non-uniform electrical properties on the device, which strongly influence the overall device performance. In some cases, such unwanted thermal gradients may even render the device unusable or less reliable due to incomplete thermal processing in some parts of the device and over-processing in others. In regions corresponding to “cold spots” on the device, important processes may not be completed, such as post-implant anneal. In regions corresponding to “hot spots”, a device may be detrimentally affected in other ways, for example from unwanted diffusion of implanted atoms into the substrate or by exceeding the thermal budget of devices formed in that region.
Methods commonly used in the art of radiation-based heating of substrates only address temperature gradients that occur on a global scale between different regions of a substrate. For example, an annular edge region of a circular substrate may have a tendency to heat more slowly than a center region of the substrate. In order to compensate for this tendency and, therefore, to improve the macroscopic temperature uniformity of the substrate, an array of lamps positioned adjacent the annular edge region may provide an increased energy output relative to centrally positioned lamps. While this approach may reduce temperature gradients across the substrate as a whole, temperature gradients within a single IC die caused by patterning effects will remain unaffected. This is because the scale of intra-die temperature gradients, which is on the order of micrometers, is far too small for different configurations of lamp arrays to correct. Moreover, modulation of the intensity of incident radiation during radiation-based heating is also unable to thermal reduce patterning effects since this method does not address the fundamentally different optical characteristics of different regions on the surface of a substrate.